An electronic device may include one or more circuit boards. A typical circuit board is a planar board that mechanically supports electronic components. The electronic components may comprise, for example, resistors, capacitors, switches, batteries, and other more complex integrated circuit components, i.e. microprocessors. The circuit board typically comprises a dielectric material, for example, a polymer material.
The circuit board may include conductive traces on the surface for connecting the electronic components to each other. As electronic circuitry has become more complex, multilayer circuit boards with at least two electrically conductive pattern layers have been developed. Typically, the different conductive trace layers of a multilayer circuit board may be connected through vertically extending vias, which comprise conductive materials, for example, metal.
A typical multilayer circuit board may comprise a plurality of core layers with bonding layers therebetween affixing the adjacent core layers together. Each core layer typically includes a dielectric layer with electrically conductive pattern layers on the opposing surfaces of the dielectric layer. Typically, during manufacture of the multilayer circuit boards, the core and bonding layers are stacked together and then heated (laminated) to cause the bonding layer to affix the adjacent core layers together.
In multilayer circuit board manufacturing, there are three typical categories for vias, i.e. a through-via, a buried via, and a blind-via. Quite simply, the through-via extends through all layers of a multilayer circuit board whereas a blind via terminates at one of the internal layers. Buried vias, on the other hand, connect two or more internal layers without providing connection to the external layers. In typical fabrication approaches, the formation of the blind-via may provide some technical hurdles for certain plating processes. In particular, the aspect ratio for the blind-via may need to be rather wide for plating techniques, for example, a 1:1 width to depth ratio. Accordingly, when traversing several layers, the blind-via may become necessarily large, which may undesirably consume circuit board real estate. In contrast, through-vias may be formed with more desirable aspect ratios, for example, 1:10 width to depth ratio.
Moreover, in typical multilayer circuit board approaches, the fabrication process includes multiple lamination steps and is onerous. Indeed, individual circuit board layers are typically laminated together sequentially, thereby adding substantial costs to the process by requiring registration and alignment at each sequential layer addition as well as multiple time consuming lamination steps.
For example, U.S. Patent Application No. 2008/0121416 to Hirai et al. discloses a multilayer circuit fabrication method. This method uses separate bonding layers with through-vias to connect circuit board layers and uses multi-step bonding methods. Also, U.S. Pat. No. 6,995,322 to Chan et al. discloses a method for making a multilayer circuit board. As in typical approaches, this method forms several separate portions of the finished multilayer circuit board in several lamination steps.